FIELD OF THE INVENTION
The present invention relates to a method pertaining to the operation of high-temperature electric furnaces. The invention also relates to a furnace of this kind.
More specifically, the present invention relates to a furnace for very high operating temperatures, viz, temperatures in the range of 1800-2000.degree. C. and higher, and also to a method of operating such furnaces. Temperatures in excess of 1800.degree. C. are achieved with the aid of electric resistor elements, for instance resistor elements comprised of stabilized zirconium dioxide.
Resistor elements for electric furnaces are made of different materials. Metallic materials can be used for temperatures up to about 1400.degree. C. It is possible to use elements of molybdenum disilicide for temperatures up to about 1850.degree. C. For temperatures higher than these temperatures, the elements may be made of graphite, stabilized zirconium dioxide and other materials. When used in oxidizing atmospheres, the resistors may be made solely of oxidic material, such as stabilized zirconium dioxide, for instance.
Neither stabilized zirconium dioxide nor resistor elements based on stabilized zirconium dioxide are electrically conductive at room temperature. The material, however, becomes conductive at higher temperatures, and marked current strengths are obtained through a zirconium dioxide element in the temperature range of 700-1000.degree. C. The conductor resistance of the material thereafter falls with rising temperatures. The material thus has a negative temperature coefficient. Consequently, in order to be able to use zirconium dioxide resistor elements in electrically heated furnaces, it is necessary to pre-heat the elements so that they are able to reach a temperature at which they are sufficiently electrically conductive to begin to work. Hitherto, this pre-heating of the elements has been achieved by using metallic resistor elements in different furnace constructions.
In furnace constructions for working temperatures above 1800.degree. C., ceramic material based on stabilized zirconium dioxide is also used for the walls, floor and ceiling of the furnace since it is found that this material is able to withstand these high temperatures better than other materials. Furnace constructions that include zirconium dioxide elements thus comprise an inner furnace chamber which is delimited by walls, floor and ceiling comprised of stabilized zirconium dioxide material. One or more resistor elements of stabilized zirconium dioxide are mounted in the inner furnace chamber. The inner walls are surrounded by an external insulation, preferably a ceramic fiber insulation. Metallic resistor elements, e.g. elements made of an iron-chromium-aluminium alloy, are embedded in this insulation at a sufficient distance from the inner furnace chamber. It is also known to arrange the latter resistor elements in an outer furnace chamber which is insulated from the surroundings. The aforesaid outer elements are used to pre-heat the furnace to a temperature at which the zirconium dioxide elements can begin to work. Because the maximum temperature to which the metallic elements can be subjected is considerably lower than the working temperature in the inner furnace chamber, the thickness of the insulation must be such as to ensure that the maximum temperature capable of being withstood by the metallic elements will not be exceeded. This results in high thermic inertia of the furnace and consequently very long pre-heating and cooling times. It is also necessary to halt the supply of energy to the metallic elements when the furnace is in operation, in order to prevent overheating of said elements.
The zirconium dioxide elements are produced in the form of straight rods or tubes. The elements have a hot zone in the center thereof and are provided at each outer end with a wire lead-in having a cross-sectional area which is larger than the hot zone. Both the hot zone and lead-ins are preferably comprised of yttrium stabilized zirconium dioxide, mutually of the same composition. To enable the transfer of energy from an external source of electric current to the elements, platinum wires are wound around the lead-ins at a suitable distance from the hot zone, and passed out through openings in the furnace chamber.
Because of the high working temperature, the supply of energy to the elements cannot be controlled in a usual manner with a temperature sensor mounted in the furnace. One method of regulating the furnace is to control the power supplied as a function of time on the basis of values obtained with experience. This method does not provide any absolute control over the temperature in the furnace chamber and results in a high degree of uncertainty, among other things because the properties of the elements vary with time.
The object of the present invention is to enable the use of zirconium dioxide elements in a manner which lengthens the useful life of said elements and of the platinum windings on the lead-ins. Another object of the invention is to enable the working temperature in the furnace chamber to be controlled and adjusted more accurately. Still another object of the invention is to provide a furnace construction which affords shorter start-up times and more rapid heating, and also more rapid cooling.